Known bolt-tightening apparatus (so called “nut runner”) for tightening bolts include electric motors and tightening members for tightening bolts. The tightening members have receiving portions for receiving heads of the bolts. With the heads of the bolts received within the receiving portions, the electric motors are driven to rotate the tightening members, so as to tighten the bolts.
One example of such apparatuses is shown in, for example, Japanese Patent Laid-Open Publication No. HEI 6-31644.
The apparatus as disclosed in the publication includes an electric nut runner and a bolt-feeding device for feeding bolts to the nut runner. The nut runner is driven by an electric motor to tighten the bolt.
FIG. 23 hereof shows an exemplary conventional method of screwing bolts 202 for attachment of an oil pan (sump) 201 to a crankcase 200 of an engine.
With the crankcase 200 turned upside down, the oil pan 201 is placed onto a bottom surface 203 of the crankcase 200 to bring apertures 205, 205 formed through the oil pan 201 into alignment with screw-threaded apertures (not shown) formed on the crankcase 200.
In this state, an operator inserts distal ends 202b, 202b of the bolts 202 into the apertures 205, 205, holding heads 202a, 202a of the bolts 202, 202 by hands 206, 206.
Then, the operator manually turns the bolts 202, 202 to screw the ends 202b, 202b into the screw-threaded apertures of the crankcase 11 by a distance of one to three times the pitch of the bolt 202. The operation of screwing the distal end 202b of the bolt 202 into the screw-threaded aperture by the distance of the one to three times the pitch will be termed “temporary tightening operation”.
Likewise, the remaining bolts 202 undergo the temporary tightening operations to screw their distal ends into the corresponding apertures of the crankcase.
After subjected to the temporary tightening operation, the respective bolts are screwed into the screw-threaded aperture through the use of such motive tool as a nut runner or the like until these bolts are tightened throughout the lengths thereof within the apertures of the crankcase.
The operation for screwing the bolt into the screw-threaded aperture by a distance equal to the overall length of the bolt to tighten the bolt within the screw-threaded aperture will be termed “final tightening operation”.
For the aforementioned method, however, the operator manually performs the temporary tightening operation with tiresomeness and troublesomeness. Moreover, the final tightening operation subsequent to such problematic temporary tightening operation requires certain time and burden. The method involving these two operations are less suitable for improving productivity of the engine.